Extrusion guns, also referred to as caulking and press-out guns, are commonly used to dispense a fluid substance stored in a dispensing cartridge. The usual structure of the caulking gun comprises a generally semicylindrical body into which the dispensing cartridge is inserted. A hand operated trigger grip mounted at one end of the body is adapted to drive a displaceable piston-rod and a circular pressure plate longitudinally along the length of the body in order to discharge the contents from the dispensing cartridge. Usually a discharge nozzle extends from the cartridge through an open end of the body opposite the trigger grip. The dispensing cartridge is placed within the caulking gun such that the pressure plate, which is mounted at the forward end of the piston-rod, is fitted within an open end of the cartridge against a plunger therein. Operation of the trigger grip moves the piston rod forward, causing the fluid within the cartridge to be discharged from the nozzle. While useful with a single-chamber cartridge, a limitation of the conventional caulking gun is that it is unable to discharge the contents of a dispenser cartridge having dual chambers within which are stored separate fluid substances.
A dual-chamber cartridge of the foregoing type is described in applicant's West German patent application DE-OS 38 14 633.9. The application discloses a container for fluid substances comprising a cylindrical housing that is divided into two chambers by means of a partition wall positioned along the longitudinal axis of the housing whereby each chamber includes a closed end having an outlet opening and an open opposite end within which is positioned a semicircular shaped plunger that is slidably displaceable towards the outlet opening. One side of the plunger includes several reinforcement ribs, and along the peripheral edge of the plunger there is a lip seal which forms an integral part of the plunger.
In West German patent application DE-GM 87 14 548 there is disclosed a manually operable caulking gun, similar to the caulking gun heretofore described, for discharging fluid substances stored inside of a dual-chamber dispensing cartridge. A transport rod and a piston rod fixed to a yoke are each fitted with a semicircular pressure plate at their respective front ends. Both rods are arranged in a parallel relation such that when operating the trigger grip mechanism of the caulking gun, both the transport rod and the piston-rod are moved forward together, thereby discharging the two components that are stored in the dual chambers of the dispenser cartridge. A disadvantage of this known caulking gun arises from the fact that there may be an expansion of the dispenser cartridge side wall due to an interior high pressure that is created when discharging the components of the dual-chamber cartridge. The cartridge appears to be particularly prone to this problem since its side wall is often made from a flexible material and also because the cartridge is usually only loosely fitted within the housing of the caulking gun, thereby leaving space for side wall expansion. As a result, the fluid contents of the dispenser cartridge may be lost by flowing back into the spaces behind the pressure plates.